Spammers tend to post in pinned long threads to evade surveillance by moderators. Several measures have been taken to prevent this:

<*>Lock those pinned threads
<*>Added URL filters to prevent putting URLs pointing to common spamming sites
<*>Warn members and put them into the "Watched members" group. Once in that group, they cannot post URL.

We cannot delete members, can we?

No, but moderators can change any member to the group of "Watched members" by using the function of "Changing warning" or flag a member as a spammer which results in the disappearance of all posts the member has made.